1. Field of the Invention
This invention relates to magneto-optical (MO) recording, which enables information to be written and erased by the use of a laser beam, and specifically, to an overwritable MO recording method and an apparatus using a single-beam source.
2. Description of the Prior art
MO recording has been put to practical use through a rewritable optical recording approach. MO recording can be classified into two categories, namely the field modulation recording method and the light power modulation recording method.
The field modulation recording method permits overwriting to be done easily by the use of a single-beam source, but has the problem that its mechanism becomes complex due to the need for a magnetic head which moves in synchronization with the laser beam. On the other hand, the light power modulation method requires no magnetic head, but has the problem that overwriting cannot be performed easily with a single-beam source.
Several studies have been done on ways to overcome the latter problem. The explanation given below is based on two examples. First, JA PUPA (published unexamined patent application) 62-175948 discloses the use of a double-layer film, emission of a high-power laser beam for writing, and emission of a lower-power laser beam for erasing. However, this approach has the problem that the mechanism becomes complex because it requires means for applying a strong initial bias field for erasing as well as means for an external bias field for writing. In addition, it is complicated to prepare a medium with two layers whose properties are well adjusted.
Hang-Ping D.Shieh et al., "Operating Margins for Magneto-Optic Recording Materials with Direct Overwrite Capability", IEEE Transactions on Magnetics, Vol. MAG-23, No. 1, January 1987 discloses the following method:
(1) An RE(rare-earth)-rich RE-TM(rare-earth-transition-metal) amorphous film with a compensation temperature not less than 40.degree. C. and not exceeding 130.degree. C. is used as a recording film. PA1 (2) A long pulse of 100 to 400 nsec is used for writing, and a short pulse of 25 to 80 nsec for erasing. PA1 (a) Short pulses have to be emitted inside the written domains. Therefore, the medium has to be turned one extra time in order to locate the written domains before the erasure step. This is not overwriting in the strict sense of the word. PA1 (b) Written domains become large because they are formed with long pulses. Therefore, the recording density cannot be increased.
The principle of writing and erasing is explained with reference to FIGS. 6 and 7. FIG. 6 shows schematically the properties of an RE-TM film with a compensation temperature higher than room temperature. As the temperature increases, the coercive force (Hc) of the RE-TM film decreases monotonically in the temperature region above the compensation temperature Tcomp, while the saturation magnetization (Ms) first increases monotonically, then decreases monotonically, as shown in the figure. In the temperature region X of FIG. 6, the coercive force Hc is low, but the saturation magnetization Ms is still high, so that the demagnetizing field Ms is also high. Therefore, the demagnetizing field may reverse the magnetization of the RE-TM film. It appears that Shieh et al. have ensured that the magnetization of the RE-TM film with property (1) is reversed by the demagnetizing field when the film is heated to temperature region X.
The main features of the method disclosed by Shieh et al. are that the demagnetizing field generated by the magnetized RE-TM film itself is utilized, and that no external field is applied. Details of the write and erasure steps are given below.
First, it is assumed that the RE-TM film is magnetized initially in an upward direction. The upward magnetization corresponds to storage of "0" and the downward magnetization to storage of "1". In this example, write means reversing the direction of magnetization from upward to downward while erasure means reversing it again from downward to upward.
Pulse L, whose duration is 100 to 400 ns, longer than that of the laser pulse L' used later for erasing, is emitted to the RE-TM film for writing (FIG. 7A). As a result, the magnetization of region P irradiated by laser pulse L is reversed by demagnetizing the H' field which is generated by the surroundings, so that "1" is written (FIG. 7B).
Next, pulse L', whose duration is short (25 to 80 ns), is emitted to the center Q of domain P (FIG. 7C). As a result, the magnetization of region Q is reversed again by demagnetizing the field H'' generated by the surroundings. Furthermore, the re-reversed domain expands (FIG. 7D) and the magnetization of the whole of domain P is finally re-reversed. Thus erasure is completed (FIG. 7E).
This method has the merit that no external field needs to be applied, but involves the following problems: